And everything I was finding suggested PC-98s ran on 100V instead of 120V...

Japan line is 100v, 50hz.

A computer after the mid-80's is almost guaranteed to be an SMPS so +-20% or more should be no problem. and after the 90's, autoranging 90-280v became ubiquitous.

You will have a problem with analog circuits (especially older AV equipment), anything motorized, heating elements, clocks, etc. A computer is almost always a non-issue, but the CRT would be questionable especially non-multisync.